The present invention relates to a coating method for the production of a corrosion-resistant sheet steel product having an outer metal-filled organic coating which does not cause cratering of cathodic electrocoat primers applied under conditions encountered in U.S. automotive electrocoating facilities.
Briefly, the coating technology to which the present invention relates is described in the literature under such terms as cathodic electrodeposition, cathodic electrophoretic coating, or e-coating. Such technology was developed in the mid-1970's and is now widely practiced in the automotive and applicance industries. The automotive industry, by way of example, adopted cathodic electrodeposition as a coating method for a number of reasons. Such reasons include the ability to obtain uniform coverage of the substrate, access to all parts of the substrate, increased corrosion protection, automation, and minimum environmental pollution, for instance. One of the disadvantages or conditions of coating through electrodeposition is that the substrate must be electrically conductive.
Although cathode electrocoat primers provide a degree of corrosion protection, paint on bare steel may not be sufficiently corrosion resistant for some applications. As a way to improve corrosion performance the steel industry turned to a zinc-rich paint system applied to only one side of a steel strip on a continuous coil coating paint line. A strong argument in support of the use of zinc pigment was the belief that such zinc would provide some galvanic protection to the underlying steel strip. In any case, a commercial product utilizing such a system is ZINCROMETAL. Such product is actually a dual coat system wherein the initial coat is a proprietary mixture of chromic acid, zinc dust and other chemicals, while the outer coating is an organic resin containing zinc powder. While ZINCROMETAL coatings appeared to satisfy the requirement for improved corrosion performance, such coating tended to show an inherent surface defect, common to all zinc coatings, when cathodic electroprimed at high voltages. By high voltages we mean voltages in excess of 250-300 volts, as typically used in the U.S. automotive industry. These surface defects had the appearance of craters or pinholes in the surface. Not only was this an appearance problem, it was also a corrosion problem. Needless to say, the subsequently applied outer coating, or cathodic electropaint, was not sufficient to mask the craters, nor to overcome the corrosion problem.